AAT4280A-3(2005) 데이터 시트보기 (PDF) - Advanced Analogic Technologies

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AAT4280A-3
(Rev.:2005)

Slew Rate Controlled Load Switch

Advanced Analogic Technologies 

AAT4280A

Slew Rate Controlled Load Switch

For maximum current, refer to the following equation:

IOUT(MAX) < ( PD(MAX) / RDS)1/2

For example, if VIN = 5V, RDS(MAX)=154mΩ and TA

= 25°C, IOUT(MAX) = 2.3A. If the output load current

were to exceed 2.3A or if the ambient temperature

were to increase, the internal die temperature

would increase and the device would be damaged.

Higher peak currents can be obtained with the

AAT4280A. To accomplish this, the device thermal

resistance must be reduced by increasing the heat

sink area or by operating the load switch in a duty

cycle manner.

High Peak Output Current Applications

Some applications require the load switch to oper-

ate at a continuous nominal current level with short

duration, high-current peaks. The duty cycle for

both output current levels must be taken into

account. To do so, first calculate the power dissi-

pation at the nominal continuous current level, and

then add in the additional power dissipation due to

the short duration, high-current peak scaled by the

duty factor.

For example, a 4V system using an AAT4280A

operates at a continuous 100mA load current level

and has short 2A current peaks, as in a GSM appli-

cation. The current peak occurs for 576µs out of a

4.61ms period.

First, the current duty cycle is calculated:

% Peak Duty Cycle: X/100 = 576µs/4.61ms

% Peak Duty Cycle = 12.5%

The load current is 100mA for 87.5% of the 4.61ms

period and 2A for 12.5% of the period. Since the

Electrical Characteristics do not report RDS(MAX) for

4V operation, it must be calculated approximately

by consulting the chart of RDS(ON) vs. VIN. The RDS

reported for 5V can be scaled by the ratio seen in

the chart to derive the RDS for a 4V VIN: 120mΩ ×

87mΩ /80mΩ = 130mΩ. De-rated for temperature:

130mΩ x (1 + .0028 × (125°C -25°C)) = 166mΩ.

The power dissipation for a 100mA load is calculat-

ed as follows:

PD(MAX) = IOUT2 × RDS

PD(100mA) = (100mA)2 × 166mΩ

PD(100mA) = 1.66mW

PD(87.5%D/C) = %DC × PD(100mA)

PD(87.5%D/C) = 0.875 × 1.66mW

PD(87.5%D/C) = 1.45mW

The power dissipation for 100mA load at 87.5%

duty cycle is 1.45mW. Now the power dissipation

for the remaining 12.5% of the duty cycle at 2A is

calculated:

PD(MAX) = IOUT2 × RDS

PD(2A) = (2A)2 × 166mΩ

PD(2A) = 664mW

PD(12.5%D/C) = %DC × PD(2A)

PD(12.5%D/C) = 0.125 × 664mW

PD(12.5%D/C) = 83mW

The power dissipation for 2A load at 12.5% duty

cycle is 83mW. Finally, the two power figures are

summed to determine the total true power dissipa-

tion under the varied load:

PD(total) = PD(100mA) + PD(2A)

PD(total) = 1.45mW + 83mW

PD(total) = 84.5mW

The maximum power dissipation for the AAT4280A

operating at an ambient temperature of 85°C is

333mW. The device in this example will have a

total power dissipation of 84.5mW. This is well

within the thermal limits for safe operation of the

device; in fact, at 85°C, the AAT4280A will handle

a 2A pulse for up to 50% duty cycle. At lower ambi-

ent temperatures, the duty cycle can be further

increased.

4280A.2005.02.1.1

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